The Arctic: The face of a new climate

Around this time every year, professionals and amateurs anxiously wait as the sea ice (i.e., frozen ocean water) in the Northern Hemisphere reaches its minimum extent for the year. This usually occurs around the second week of September. As the winter season picks up, sea ice then becomes more extensive as the Arctic becomes shaded from incoming sunlight. Several decades ago, Arctic sea-ice cover was thought to be in such a near-steady seasonal cycle, reaching an area of roughly 15 million square kilometers each March, and retreating to 7-8 square kilometers each September.

However, during the last few decades there has been a downward trend in the amount of sea ice that survives during the summer months. This year, Arctic sea ice extent hit a record low, and did so a few weeks before the melting season usually ends. The typical month-to-month pattern, as well as the steady downward trend, is shown in the following figure. The blue and orange lines show the annual cycle of sea ice extent for 2011 and 2007, respectively. The red line shows the 2012 sea ice pattern up until the time of this post. The black and brown lines show the “average” conditions for the 1980 and 1990 decades.

Data from http://www.ijis.iarc.uaf.edu/en/home/seaice_extent.htm

There is a noticeable decline in sea ice extent in all months, but especially so during the warm season. August and September are especially interesting months since several models show that the Arctic ocean could become nearly ice-free during the summer within just a few decades as global warming continues. Thus, these months will define the transition from perennial to seasonal ice cover in the Arctic. This is important for a number of reasons. Sea ice is a major component of polar ecosystems. Plants and animals at all trophic levels use sea ice as their habitat. Moreover, such rapid decline already has changed the extent of human activity in the Arctic (including shipping, military presence, oil, gas and mineral exploitation, and fisheries).

The long-term reduction in overall Northern Hemisphere sea ice extent is true for most individual regions as well, including Hudson Bay, Baffin Bay/Labrador, the Greenland Sea, the Arctic Ocean, the Canadian Archipelago, and others. This decline has been attended by a transition towards a thinner, younger ice cover (as opposed to predominately multi-year ice which survives for two or more years). Thinner ice in spring fosters a stronger summer ice-reflectivity “feedback loop” through earlier formation of open water areas.

This reflectivity feedback, also known as the “ice-albedo feedback”, operates because ice/snow is more reflective than darker ocean or melt water. Thus, if something happens to make the Arctic warmer, one would expect reduced ice cover, which would then decreases the amount of sunlight that is reflected away. More solar energy is absorbed by the oceans, which then exacerbates the warming or melting. Without this feedback, the amplitude of the seasonal cycle in ice extent (the change from February or March through September) would be smaller than is observed. But it’s also important on the longer timescales as well, reinforcing the influence of greenhouse gases, and at least partially responsible for the fact that Polar Regions tend to warm faster than the global average in observations and in future simulations of greenhouse-induced climate change (I have less confidence than others in the scientific community that it is the predominant mechanism involved behind this so-called “Arctic amplification,” but it is at least a key regional driver of the future Arctic climate).

There is a clear seasonal structure in this feedback. Increased absorption of sunlight in open waters in summer impact the survival of the summer sea ice, but there is very little temperature amplification (compared to lower latitudes) in the summer, since much of the energy is deposited into melt or evaporation. However, this extra energy gained by the ocean must be released back to the atmosphere before sea forms in the autumn and winter. This leads to a shorter ice growth season and higher autumn air temperatures. Much of the Arctic winter sea ice retreat is thus not ice that has melted; this sea ice never froze.

The evolution of albedo, or reflectivity, during a typical year is shown below. Higher albedo values on the vertical axis indicate more energy reflected away, while lower values indicate more energy absorbed. The next figure shows the evolution of temperature anomalies by year and month, based on the JRA-25 atmospheric reanalysis, a product of the Japan Meteorological Agency. The anomalies are the difference between the temperatures at whatever time you are looking at and the average of the period 1979–2010. You can visually see a trend toward warming temperature, as well as greater warming in the spring and autumn months.

Time series of the evolution of seasonal ice albedo. Seven phases of melt are illustrated. From Perovich and Polashenski, 2012

Evolution of Arctic Temperature Anomalies by year and month (see Text). From Stroeve et al, 2011

The causes of the strong decrease in Arctic sea ice extent are complex. No one has successfully explained the strong trend with just natural variability alone. There is a thermodynamic forcing component that is linked to increases in greenhouse gases and warming temperatures. However, natural variability is large in the Arctic, and warming “pre-conditions” the ice toward less extensive and thinner conditions. This ice can then be exposed to storms that are readily capable of exporting large amount of ice, which will then be fated to melt away. For example, a big player in the 2007 sea ice minimum (now second lowest on record) was an unusually high sea level pressure over the Beaufort Sea and Canada Basin, and anomalously low pressure over eastern Siberia (called a summer Arctic Dipole pattern). This led to strong southerly winds and warm temperatures, transporting ice away from the coasts of Siberia and Alaska, and from the Arctic Ocean to the North Atlantic. Such patterns have been observed before however, so without the extensive thin, first-year ice, the atmospheric pattern would not have produced such a radical reduction in observed sea ice extent. The dramatic 2012 ice loss observed so far was at least aided by a cyclone that helped break up ice. Thus, “weather” will still be superimposed on a ever-steepening downward trend in Arctic sea ice.